Conventionally, there has been known a configuration in which a magnetic fluid seal is provided in an annular gap between a shaft and a housing that rotate relative to each other, and a filter is further provided in an air hole that communicates two regions separated by the magnetic fluid seal (see Patent Literatures 1 and 2, for example). This configuration enables the removal of the foreign matters contained in a gas in the annular gap, while keeping the friction torque low. One of the problems with this configuration, however, is that because the magnetic fluid seal and the filter are provided separately, a configuration for providing the magnetic fluid seal together with the filter becomes relatively large. Moreover, while the pressure difference between the two regions can be eliminated with the use of the air hole, the magnetic fluid is still likely to disperse when, for example, a relatively high pressure acts on the magnetic fluid seal.
In addition, the foregoing configuration employs a conventional magnetic fluid seal that retains the magnetic fluid in a small annular gap between the shaft and a pole piece provided in the housing. Therefore, in this configuration, when re-inserting the shaft, that has been removed from the housing, into the housing, the position of the shaft with respect to the housing (the position of the shaft in the axial direction and the radial direction) needs to be adjusted with high precision. Note that because the removed foreign matters accumulate in the filter, the filter needs to be replaced. Depending on where the filter is installed within the housing, the shaft may need to be removed from the housing for replacing the filter. Therefore, according to such a configuration in which the conventional magnetic fluid seal is employed, it is difficult to position the shaft in the axial direction and radial direction when inserting the shaft, and as a result, a work to replace the filter also becomes difficult.
Incidentally, a magnetic fluid seal that stably retains a magnetic fluid even during an occurrence of an eccentric motion of the shaft is proposed (see Patent Literature 3, for example). In this magnetic fluid seal, however, an impact of the fluid pressure in the annular gap on the magnetic fluid is not taken into consideration; hence, there is a possibility of the magnetic fluid being dispersed when a relatively high fluid pressure is applied.